United States vice-president Joe Biden has sparked intense speculation that Barack Obama will endorse same-sex marriage if he wins a second term as president.

Mr Biden, a 69-year-old Catholic, said on national television that he is "absolutely comfortable" with gay marriage, a stance that goes well beyond the president's suggestion that his views are evolving on the issue.

"What this is all about is a simple proposition: who do you love?" Mr Biden said.

"I am absolutely comfortable with the fact that men marrying men, women marrying women and heterosexual men marrying women are entitled to the same exact rights."

The vice-president's remarks were followed up by Mr Obama's education secretary, Arne Duncan, who went on cable television, also supporting not just equal rights for same sex couples, but using the word marriage.

Gay marriage has been a delicate topic for the Obama administration, which is why it captured national attention when Mr Biden became the highest ranking White House official to express support for it.

It is now legal for same-sex couples to wed in six US states, as well as Washington DC, but the marriages remain unrecognised by the government.

Mr Obama has already abolished the military's "don't ask don't tell" policy, ushering in the chance for soldiers to be openly gay.

Polls show a majority of Americans now support gay marriage.

But the vice-president's comments up the pressure on Mr Obama to take a clearer stance and to make gay marriage part of the Democratic platform at its convention in September.

But White House press secretary Jay Carney is not obliging the media.

Is there anyone who doesn't think that president Obama believes in gay marriage? Nobody. I mean everybody assumes that he's for gay marriage. I assume that, we all assume that.

Democratic strategist Howard Wolfson

"I can tell you that the president has spoken about this and that his views have not changed and I have no update to give you on them," he said.

The general belief is that Mr Obama is reluctant to make the announcement before an election that might be decided in states like Ohio, Pennsylvania and Virginia, where support of gay marriage could lose him votes.

In a sign of how politically contested the issue still is, North Carolina - a potential swing state - is about to vote on a measure to ban same-sex marriage.

Democratic strategist and deputy mayor of New York City, Howard Wolfson, is one who thinks the president is relying on the political cover of aides to signal his intentions.

"I mean is there anyone who doesn't think that president Obama believes in gay marriage? Nobody. I mean everybody assumes that he's for gay marriage. I assume that, we all assume that," he said.

"Having vice-president Biden go out and basically affirm that, having members of the cabinet come out and affirm that, gives a sense to people that, yes, the president is there, he made not be able to do it right now, for political reasons, but his heart in the right place and maybe there's an opportunity for him to do it down the line."

Gay rights advocates like Darlene Nipper from the National Gay and Lesbian Taskforce see progress.

"Hopefully this is a sign and a signal the next person we'll hear this from will be president Obama," she said.